1. Field of the Invention
The present invention relates to an oscillator, and more particularly to an oscillator with a surface acoustic wave filter in its feedback path.
2. Description of Prior Art
Recently there have been developed CATV systems which use a tuner adapted for receiving a plurality of broadcast channels. These tuners use a so-called up/down heterodyne tuner which is better able to eliminate IF interruption. The up/down heterodyne tuner first converts the carrier frequency of the received signal to an intermediate frequency (IF) higher than the carrier frequency, and then converts the IF to a predetermined frequency lower than the carrier frequency.
Up/down heterodyne tuners adapted for CATV systems are generally constructed as shown, for example, in FIG. 1. In FIG. 1, broadcast signals (input signals) with predetermined carrier frequencies among a frequency band ranging, e.g., from 50-550 MHz are provided to an input terminal 1 from a receiving antenna (not shown). The input signals are introduced to a first mixer 3 through an input circuit 2. In first mixer 3, the input signals are converted from their carrier frequencies to a predetermined intermediate frequency (IF) by using a first local oscillation signal applied from a first local oscillator 4 to first mixer 3. The IF signal may be, for example 612.75 MHz, which is higher than the highest frequency, 550 MHz, of the frequency band. The IF signal is then input to a first IF filter 5, an IF amplifier 6 and a second IF filter 7, where its level and tuned characteristic are adjusted to the desired level and characteristic by the circuit. The IF signal output from second IF filter 7 is input to a second mixer 8. In second mixer 8, the IF signal is converted from 612.75 MHz to a frequency, for example, 61.25 MHz, which is lower than the IF and which corresponds to one of the predetermined channels set for television receivers. The frequency conversion in the second mixer 8 is performed by a differential operation between the IF signal and a signal applied from a second local oscillator 9 to second mixer 8.
The signal output from the second mixer 8 is introduced to an output terminal 11 of the up/down heterodyne tuner through an output circuit 10. The output signal on output terminal 11 is supplied to a television receiver (not shown) as its so-called antenna input signal.
In the above described up/down heterodyne tuner, the selection of a desired channel is made by varying the frequency of first local oscillator 4. The output signal of this up/down heterodyne tuner is assigned to a frequency corresponding to any one of channels 1, 2, 3 and 5 in Japan, or channels 2, 3, 4 and 5 in the U.S.A. and Korea. The selection of the frequency accords to the frequency of the output signal of second local oscillator 9. Therefore, the output of second local oscillator 9 is varied in frequency in a band of 704-790 MHz in Japan, or 668-696 MHz in the U.S.A. and Korea, in general.
This up/down heterodyne tuner for CATV systems uses a Colpitts oscillator as second local oscillator 9. Colpitts oscillators contain L/C resonance circuits comprised of an inductor L and a capacitor C in a feedback path for feeding back an oscillation output of the oscillator to its input side with a prescribed phase relation. However, a Colpitts oscillator of this type has a disadvantage in that its output frequency easily fluctuates as a result of changes in temperature and humidity or the bias voltage supplied to the circuit. Therefore, the output frequency of the up/down heterodyne tuner for CATV systems becomes unstable when an L/C resonance circuit is used for its second local oscillator 9, and interferes with broadcast reception by the television receiver.
Many attempts have been made to improve the stability of the output frequency of the up/down heterodyne tuner. In these attempts, an oscillator using a surface acoustic wave filter (referred to as a SAW filter hereinafter) in its feedback path has been developed, so that an up/down heterodyne tuner with an oscillator of the described type as its second local oscillator 9 is able to produce an oscillation output with a relatively stable frequency. The SAW filters are constructed principally as shown in FIG. 2. That is, interdigital transducer electrodes (IDT electrodes) 20a and grating reflectors (GRs) 20b respectively, made of, e.g., aluminum film, are laid on a base 20c, e.g., made of the lithium-tantalate LiTaO.sub.3 for mounting IDT electrodes 20a and GRs 20b and transmitting acoustic waves therebetween.
Oscillators using the SAW filter create a problem in that the SAW filter is itself apt to be easily damaged by the oscillation signal in a feedback path. This problem is caused by the energy of a surface acoustic wave in the SAW filter concentrating and accumulating on specified portions of the SAW filter. The surface acoustic wave arises on base 20c as a standing wave, as illustrated by solid lines shown in FIG. 2. In FIG. 2, the solid lines also show the distribution of displacement of base 20c due to the standing wave, while the dotted lines show the distribution of mechanical strain due to the standing wave in base 20c. That is, the energy of the surface acoustic wave, which is sometimes as high as the exciting power for the oscillator, is accumulated in the SAW filter due to the standing wave. The energy is concentrated on the surface of base 20c of the SAW filter. An excessive concentration of energy causes a so-called migration phenomenon in IDT electrode 20a or GRs 20b fixed on the surface of base 20c of the SAW filter. The migration phenomenon is the phenomenon that voids or hillocks occur in metal films like electrodes formed on dielectric bases, as known, e.g., in the field of semiconductor device techniques. It is believed that the voids or hillocks occur as a result of so-called metal fatigue phenomenon having excessively advanced due to repetition of mechanical strains in metals.
In the SAW filters, the voids or hillocks increase the electric resistance of IDT electrode 20a or GRs 20b. Further, part of the oscillation signal energy is consumed in base 20c as Joule equivalent according to the piezoelectric effect in base 20c. The increased electric resistances and the consumed Joule equivalent energy further promote the damage of the SAW filters.
Therefore, oscillators using SAW filters have been conventionally used only when the oscillators are not excited to a high energy oscillation state. In other words, the oscillators must be operated at a low energy state. The exciting energy is very critical and it is difficult to maintain the oscillation at low energy levels.